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Wireless Link Budget Analysis How to Calculate Link Budget for Your Wireless Network

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Wireless Link Budget Analysis How to Calculate Link Budget for Your Wireless Network Wireless Link Budget Analysis How to Calculate Link Budget for Your Wireless Network Whitepaper Radio Systems How far can it go and what will the throughput be? These are the two common questions that come up when designing a high speed wireless data link. There are several factors that may impact the performance of a radio system. Available and permitted output power, available bandwidth, receiver sensitivity, antenna gains, radio technology, and environmental conditions are some of the major factors that may impact system performance. For large scale network deployments, a detailed site survey and network design are highly recommended. This paper will attempt to provide the reader with an overview on how a link budget is calculated. Line-of-Sight (LOS) Link Budget To limit the scope of this paper, only line-of-sight links with sufficient Fresnel Zone clearance will be considered. The following equation shows the basic elements that need to considered when calculating a link budget: Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB) If the estimated received power is sufficiently large (typically relative to the receiver sensitivity), the link budget is said to be sufficient for sending data under perfect conditions. The amount by which the received power exceeds receiver sensitivity is FSPL (dB) called the link margin . Distance 900MHz 2.4GHz 5.8GHz 1km Free-Space Path Loss 91.53 100.05 107.72 2km 97.56 106.07 113.74 In a line-of-sight radio system, losses are mainly due to free-space path loss (FSPL ). FSPL is proportional to the square of the distance between the transmitter and 3km 101.08 109.60 117.26 receiver as well as the square of the frequency of the radio signal. In other words, 4km 103.58 112.10 119.76 free-space path loss increases significantly over distance and frequency. 5km 105.51 114.03 121.70 10km 111.53 120.05 127.72 ͨ_! ͦ ͍̀͊͆ ̼͘ Ɣ 10 log ͥͤ 20km 117.56 126.07 133.74 30km 121.08 129.60 137.26 Other losses in a radio system to consider are due to antenna cabling and connectors, 40km 123.58 132.10 139.76 which are already accounted for in Tranzeo’s all-in-one units with integrated antennas. 50km 125.51 134.03 141.70 However, for units with external antenna, 0.25dB loss per connector and 0.25dB loss for every 3-ft of antenna cable should be included in the link budget calculations. For Table 1: Free-Space Path Loss (FSPL) a radio system with a 3-ft LMR400 cable and 2 connectors, 0.75dB loss should be included . The FSPL equation can be further simplified as follow: where: ͨ_ ͍̀͊͆ ̼͘ Ɣ 20 log ͘ ƍ20log ͚ ƍ20log f is signal frequency in Hz ͥͤ ͥͤ ͥͤ d is distance in meters ( m) ͍̀͊͆ ̼͘ Ɣ 20 log ͥͤ ͘ ƍ20log ͥͤ ͚ Ǝ 147.55 c is the speed of light in a vacuum ( 3 x 10 8 m/s ) Normally, distance is measured in km or miles and frequency in MHz , in this case the above equation becomes: where d is in km and f in MHz ͍̀͊͆ ̼͘ Ɣ 20 log ͥͤ ͘ ƍ20log ͥͤ ͚ ƍ 32.45 ͍̀͊͆ ̼͘ Ɣ 20 log ͥͤ ͘ ƍ20log ͥͤ ͚ ƍ 36.58 where d is in miles and f in MHz Table 1 shows some calculated FSPL values for 900MHz, 2.4GHz, and 5.8GHz links. Tranzeo Wireless Technologies Inc. 19473 Fraser Way, Pitt Meadows, BC, Canada V3Y 2V4 • T: 604.460.6002 • F: 604.460.6005 • Toll Free: 1.866.872.6936 • www.tranzeo.com © 2010 Tranzeo Wireless Technologies. All rights reserved. Tranzeo and the Tranzeo logo are registered trademarks of Tranzeo Wireless Technologies Inc. TR3014-01 All other trademarks mentioned herein are the property of their respective owners. Wireless Link Budget Analysis | How to Calculate Link Budget for Your Wireless Network PAGE 2 Multipath and Fade Margin Data Rate SNR Modulation & Encoding Scheme Multipath occurs when waves travel along different paths and cause (Mbps) (dB) unwanted interference with the waves travelling on the direct line-of- BPSK 1/2 6 8 sight path. This is normally referred to as fading. A rare worst case occurs when waves travelling along different paths end up completely BPSK 3/4 9 9 out of phase and cancel each other. One way to overcome this problem QPSK 1/2 12 11 is to transmit more power, or have enough link margin. In some cases, relocating or repositioning the antennas slightly may reduce the QPSK 3/4 18 13 impact of multipath. 16-QAM 1/2 24 16 ͆͢͟͝ ͇͕ͦ͛͢͝ Ɣ ͙͙͙͌͗ͪ͘͝ ͙͊ͣͫͦ Ǝ ͙͙͙͌͗ͪ͝ ͍͙ͧͨͪͨͭ͢͝͝͝ 16-QAM 3/4 36 20 64-QAM 2/3 48 24 Fading due to multipath can result in a signal reduction of more than 30dB, and it’s highly recommended that adequate link margin is 64-QAM 3/4 54 25 factored into the link budget to overcome this loss when designing a Table 2: Data Rates vs. Minimum SNR wireless system. Signal-to-Noise (SNR) Modulation techniques not only determine system bandwidth and channel capacity, they also determine system reliability. It’s always a trade-off between data rates and distance. More efficient modulation techniques such as 64-QAM require greater SNR, but less efficient techniques such as BPSK require less SNR, and therefore are more resilient to channel noise. For the purposes of link budget analysis, the most important aspect of a given modulation technique is the Signal-to-Noise Ratio (SNR) necessary for the receiver to achieve a specified level of reliability in terms of Bit Error Rate (BER). The amount of extra RF power radiated to overcome this phenomenon is also referred to as fade margin. The exact amount of fade margin ͍͈͌ Ɣ ͙͙͙͌͗ͪ͘͝ ͙͊ͣͫͦ Ǝ ͕͙̽͜͢͢͠ ͈͙ͣͧ͝ required depends on the desired reliability of the link, but a good rule- of-thumb is to maintain 20dB to 30dB of fade margin at all times. Table 2 shows the minimum SNR required for the different modulation and encoding schemes. Site surveys should be conducted in all Table 3 shows the Rayleigh Fading Model, which highlights the deployment locations to ensure that sufficient SNR can be achieved to relationship between the amount of available link margin and link meet the desired data rates. availability as a percentage of time. Time Availability (%) Fade Margin (dB) Why Tranzeo 90 8 Tranzeo offers a complete family of 802.16d (802.16-2004) WiMAX products for 3.5GHz, 3.65GHz, and 5.8GHz spectrums 99 18 including indoor and outdoor Subscriber Units and Pico Base 99.9 28 Stations. As well, we offer a comprehensive line of 900MHz, 2.4GHz, 4.9GHz, and 5.8GHz 802.11a/b/g and 802.11n standards- 99.99 38 based WiFi products including Routing Access Points, CPE’s, Full- Duplex PtP Bridges, and advanced Mesh Routers and Access 99.999 48 Points for complete turnkey solutions. Table 3: Rayleigh Fading Model Tranzeo Wireless Technologies Inc. 19473 Fraser Way, Pitt Meadows, BC, Canada V3Y 2V4 • T: 604.460.6002 • F: 604.460.6005 • Toll Free: 1.866.872.6936 • www.tranzeo.com © 2010 Tranzeo Wireless Technologies. All rights reserved. Tranzeo and the Tranzeo logo are registered trademarks of Tranzeo Wireless Technologies Inc. TR3014-01 All other trademarks mentioned herein are the property of their respective owners. Wireless Link Budget Analysis | How to Calculate Link Budget for Your Wireless Network PAGE 3 Design Example We’ll use the TR-5plus-24 radio system as an example to illustrate how a link budget is calculated using the information presented in this paper. Example: Distance: 5 km Frequency: 5.8GHz Link Type: Point-to-Point (PxP) Line-of-Sight: Yes Radio Systems: 2 x TR-5plus-24 Link Budget Analysis: First, we need to calculate the received power. In order to do that, we need to know the transmitter power and antenna gains of the radio systems that we’re using, which is normally documented in the datasheet of the radios and antennas. Since we’re using 2 x TR-5plus-24, then the link budget in both directions is expected to be symmetric. This is true because the transmit power of both radios is expected to be the same. In cases where 2 types of radio systems with different transmit power levels are used, then 2 link budgets, one for each direction, will need to be analyzed. Consulting the TR-5plus-24 datasheet, the Tx power is +23dBm , and the internal antenna gain is 24dBi . Since this product features an integrated antenna, we can assume negligible loss for cabling and connectors. Since this is a point-to-point link with sufficient Fresnel Zone clearance, then the only losses that we need to consider when calculating the received power is the free-space-path loss (FSPL). The FSPL at 5 km is 121.70dB for 5.8GHz as per Table 1. The received power can then be calculated as follow: Received Power (dBm) = Tx Power (dBm) + Tx Antenna Gain (dBi) + Rx Antenna Gain (dBi) – FSPL (dB) = 23 + 24 + 24 – 121.70 = -50.70 The required SNR to achieve 54MBps data rate is 25dB, as per Table 3. Now that we know what the received power and the minimum SNR, we could determine what the maximum channel noise is: Maximum Channel Noise (dBm) = Received Power (dBm) – SNR (dB) = -50.70 – 25 = -75.70 Next, we need to ensure that we have enough link margin for a reliable link.
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